Portable communication devices such as cellular-type telephones or other communication devices are becoming more widespread. A portable communication device includes one or more power amplifiers for amplifying the power of the signal to be transmitted from the portable communication device.
With the decreasing size of portable communication devices, power efficiency is one of the most important design criteria. Reducing power consumption prolongs power source life and extends stand-by and talk time of the portable communication device.
A portable communication device may employ a constant or a non-constant envelope modulation methodology. A non-constant envelope modulation scheme is typically implemented with a linear power amplifier. The entire amplitude and phase modulated waveform is provided to the input of the power amplifier and the power amplifier amplifies the combined signal. In a non-constant envelope modulation scheme, “power control” can be implemented as a “slow loop” regulating the gain of the power amplifier or adjusting the input amplitude to compensate for gain variation in the power amplifier that occurs due to process and temperature variations. Unfortunately, a linear power amplifier is significantly less efficient than a nonlinear power amplifier and, as such, consumes more power.
In the case where both a constant envelope modulation methodology and a non-constant envelope modulation methodology are employed, such as in a communication device that operates using the Global System for Mobile Communication (GSM) and the Enhanced Data Rates for GSM Evolution (EDGE) communication formats, the same power amplifier should be used for both signals. The GSM system provides a slightly higher output power and uses a constant-envelope modulation methodology. The EDGE system uses a non-constant-envelope modulation methodology. If a linear power amplifier is used to implement EDGE, then the power amplifier is less efficient when operated in GSM mode. This is why it is desirable to find a way to make a non-linear power amplifier work in EDGE mode.
Polar modulation is a known technique of performing non-constant envelope modulation using a nonlinear power amplifier. In polar modulation, a phase modulated input signal is applied to the radio frequency (RF) input to the power amplifier. The output power of the power amplifier is adjusted at the rate of the amplitude modulation to recompose the modulated waveform at the output of the power amplifier.
A GSM system has traditionally been implemented using a nonlinear power amplifier, with the “power control” implemented as a (slow) gain modulation in the power amplifier. A “power control” signal is supplied to the power amplifier from the baseband subsystem to implement the time-slotting (ramp up power at the beginning of the time slot, ramp it down at the end) of the communication protocol using this slow gain modulation. One prior attempt at implementing a power amplifier in the EDGE system using polar modulation increases the performance of the “power control” signal, so that the power amplifier output power can be changed rapidly to create the modulation and to create the power control (i.e. there is still the slow ramp up and ramp down at the edges of the slot, but the faster modulation is also added in the middle). In this manner, the power amplifier can still be used in GSM mode by applying a signal to the “power control” port with only the ramping signals, while also performing polar modulation in EDGE mode.
There are two kinds of polar modulation: open-loop and closed-loop. In open loop, there is no feedback path for the power amplifier output. In closed-loop, feedback on the amplitude and phase paths is used to measure the output amplitude and phase. The measured amplitude and phase are compared to a desired signal, and then an amplitude and gain correcting mechanism is used to minimize any discrepancy. Such an implementation is difficult while maintaining a very wide bandwidth, meeting noise requirements and preventing the system from becoming unstable and oscillating under output mismatch, for example, in the presence of a voltage standing wave ratio (VSWR).
In such a system, the phase modulation is typically applied directly to the signal input of the power amplifier. The phase can be controlled using a phase correction feedback loop. One of the challenges when implementing a so called “closed-loop polar modulation” technique is that the amplitude portion of the power amplifier output signal must be removed prior to providing the output signal to the phase correction loop.
A radio frequency (RF) limiter can be utilized in many applications where it is desirable to remove amplitude modulation from an input RF signal. Such applications can include the above-mentioned polar modulation, phase correction loops, phase modulation recovery, and other suitable applications. In polar modulation, a limiter can be used to remove amplitude modulation from an input RF signal while preserving the desired phase modulation for transmittal to a polar mode power amplifier. In a phase correction loop, a limiter can be used to remove amplitude modulation from an output signal for use in a phase correction feedback loop, such as to reduce the AM/PM distortion, which may otherwise occur in a phase detector utilized in the feedback loop. In such systems, it can be desirable for the limiter to reproduce the input phase accurately at the output over a wide range of input amplitude levels, so that a change in the input amplitude level does not alter the relationship between the input and output phase.
FIG. 1 is a schematic diagram illustrating a simplified prior art limiter 100. The limiter 100 comprises one or more variable gain amplifiers 101, an optional fixed-gain amplifier 102, an envelope detector 103, an optional buffer 104, and an error amplifier 105. The variable gain amplifiers 101, if more than one amplifier is used, can be connected in series with one another and can be of the same type or different types. The output of variable gain amplifier 101 is provided to an optional fixed-gain amplifier 102. The fixed-gain amplifier 102 can be used if the output of the variable gain amplifier 101 has less than desirable amplitude, such as to drive an envelope detector. The output of fixed-gain amplifier 102 is provided to an input of the envelope detector 103 and to an input of the optional buffer 104.
The envelope detector 103 responds to the amplitude of the signal output from the fixed-gain amplifier 102 and generates an envelope signal representing the detected amplitude. The envelope signal representing the detected amplitude is provided to the error amplifier 105. The error amplifier 105 compares the detected envelope signal with a signal representing a desired amplitude at its non-inverting input and generates a gain control signal which is provided to the variable gain amplifier 101. In an example in which multiple variable gain amplifiers are provided, each of the variable gain amplifiers 101 receives the same gain control signal. In another example, the variable gain amplifiers 101 can receive multiple gain control signals. The feedback loop closed by the error amplifier 105 can adjust the gains of variable gain amplifiers 101 in such a way as to keep the output of the detector 103 the same as the desired signal envelope resulting in a limited signal having no AM component. Optional buffer 104 can reproduce this signal to generate an output signal. If optional buffer 104 is omitted, the output of fixed-gain amplifier 102 or of variable gain amplifiers 101 can be used as the output signal.
FIG. 2 is a schematic diagram illustrating a prior art variable gain amplifier which can be used to implement the variable gain amplifier of FIG. 1. The variable gain amplifier 200 comprises an amplifying device in the form of an amplifying transistor 201, variable resistor 202, and load resistor 203. A capacitor 204 can represent an unwanted parasitic, which may be present in the circuit. The amplifying transistor 201 is configured to receive an RF input signal and is connected to the variable resistor 202 so that the resistance of the variable resistor 202 controls the gain of the RF input signal through amplifying device 201. The amplified signal is provided as the output of the variable gain amplifier 200. An example of the variable gain amplifier 200 can use a metal oxide semiconductor field effect transistor (MOSFET) device as the amplifying device 201. In such an example, the amplifying device 201 is configured so that the RF input signal is coupled to the gate terminal of the amplifying device 201 while the variable resistor 202 is connected to the source terminal of the amplifying device 201. Other configurations, such as using a bipolar transistor as an amplifying device 201, using compound devices such as cascode connected transistors for the amplifying device 201, or other suitable configurations are also possible. Optional load resistor 203 can be used to generate an output voltage from the current of amplifying device 201.
The capacitor 204 represents a common parasitic, or unwanted capacitance, which can be present. The capacitor 204 may be a junction capacitance associated with the amplifying device 201, capacitance of a metal trace on an integrated circuit (IC) or printed circuit board (PCB), a capacitance associated with the variable resistor 202, or another undesirable capacitance. The capacitance 204 can be detrimental if the variable gain amplifier 200 is constructed so that the gain of the amplifying device 201 is controlled by an impedance determined by both the variable resistor 202 and the capacitor 204. For example, if the capacitor 204 and the variable resistor 202 are each connected between the amplifying device 201 and an AC ground, the gain through amplifying device 201 can be inversely related to the input impedance of the variable resistor 202 in parallel with the capacitor 204. Since this impedance can be a complex number, the phase of the gain can be related to the phase of the complex number arctan(b/a) where a and b are the real and imaginary parts of the complex number, respectively. As the gain of the variable gain amplifier 201 is adjusted using the variable resistor 202, the phase of this impedance can change, causing AM/PM distortion. A representative example of such a phase change is illustrated in FIG. 3, in which the waveform 301 represents the phase of the impedance Z as the resistance value R of variable resistor 202 is changed.
Therefore, it is desirable to have an RF limiter that minimizes distortion.